This invention relates to a longwall mining system and particularly to extensible roof support units supported at their outer end by the longwall.
Longwall mining systems presently in use commonly utilize mining machines having single or double drum shearers operating above an armored conveyor along a face which is typically six hundred feet long. The roof is supported in the vicinity of the longwall face by self-advancing fixed cantilever beam supports and the roof is caved in behind the supports. This system is satisfactory in geographic areas where the roof is good. However, in those areas where the roof condition is bad the roof tends to collapse at the juncture of the roof and the wall face where insufficient support is provided between the end of the cantilever beam supports and the wall face.
When a two pass shearer makes a pass, it exposes an unsupported lengthwise extending portion of the roof immediately adjacent the wall and extending the full length of the wall. The fixed cantilever beam supports cannot advance into the newly cut face until after the second pass is made. Even then the cantilever beam supports lag a considerable distance behind the continuous mining machine because the conveyor sections must include a transitional length several sections long.
Telescoping beams have been used for roof support systems but have not proven satisfactory because extremely long cantilever lengths are required which must be of considerable strength to carry the heavy roof loads in the vicinity of the longwall face. When a cantilever beam, ie a beam supported at one end and unsupported at the other end, is loaded along its entire length, the maximum bending moment that it sustains is four times the maximum bending moment sustained by a simply support beam, ie a beam supported at each end, which carries the same load. The only known attempt to provide support for a roof system at the wall end is disclosed in U.S. Pat. No. 2,716,025. However, the system disclosed in this patent has evidently not found general acceptance. There appear to be several reasons for this. One is that a belt conveyor is used which cannot be advanced in sections, as can an articulated conveyor. Further, the mining machine itself is used to provide temporary support for the roof when the machine is passing by, which presupposes an exactness of roof support elevational alignment not common in mining. In addition, it appears that the relatively movable secondary portion of the roof support must be carried by a separate base unit which results in clearance problems. Finally, the operation appears to be manual and there is no suggestion of how it can be adapted to current mining procedures.
The present longwall mining system overcomes these and other problems in a manner neither disclosed nor suggested in the known prior art.